Switch to gotestsum

### What problem are you facing?

We currently use https://github.com/jstemmer/go-junit-report to create Sonobuoy compatible junit reports from our tests. It seems like there are several known issues with go-junit-report that are languishing; including that it can't handle `cmp` output correctly.

### How could conformance help solve your problem?

We could switch to https://github.com/gotestyourself/gotestsum/, which appears to be the new hotness (and better supported).

Start by locating where go-junit-report is invoked to generate Sonobuoy-compatible JUnit reports. Read the surrounding test or reporting configuration, then review gotestsum's report options and verify that the resulting output remains compatible with Sonobuoy, including cmp output handling.

Written by the indexing model from the issue text.